1. Field of the Invention
The present invention relates to a liquid-crystal display device which is easy to reduce thickness and weight and excellent in display quality and which can be used both in an external light mode and in an illumination mode.
2. Description of the Related Art
A liquid-crystal display device which includes a semi-transmission reflection layer such as a half mirror and which can be used both in an external light mode and in an illumination mode has become popular widely in a portable apparatus such as a portable personal computer or a cellular phone. Reduction in weight achieved by reduction in size and thickness of the portable apparatus has been strongly demanded in order to improve its portability more greatly. There is, however, the actual situation that reduction in thickness and weight of the device as a whole has reached its practical limit because even in the case where a related-art side light type light pipe excellent in thinness is used in a backlight unit to obtain good visibility in an illumination mode, the thickness of the backlight unit is generally not smaller than 2 mm.
In consideration of such circumstances, there has been proposed a reflective liquid-crystal display device which can be used both in an external light mode and in an illumination mode and in which light is made incident on a side surface of a liquid-crystal display panel having a light output means-including optical film provided on a visual side surface of a liquid-crystal cell so that transmitted light in the panel is reflected by the light output means to illuminate the panel (Unexamined Japanese Patent Publication No. 2000-147499). This proposal is made so that a system for illuminating the liquid-crystal display panel can be put into practice by use of the optical film far thinner than the side light type light pipe to thereby achieve reduction in thickness and weight of the device.
There is, however, a problem that display luminance is low because the efficiency of incidence of light on a side surface of the panel is lowered remarkably with the reduction in thickness of cell substrates carried out in the recent years. There is also a problem that it is difficult to make the distribution of luminance uniform when the transmission distance of light in the direction of the side surface is long compared with the thickness of the cell substrates, that is, when the area of the cell substrates is relatively large. These problems arise also in the liquid-crystal display device which includes a semi-transmission reflection layer and which can be used both in an external light mode and in an illumination mode.